use pump to pipe sockets

Ensures that destination socket close or destroy also does the same for
the source socket.

lib/ClientManager.test.js

@@ -46,6 +46,12 @@ describe('ClientManager', () => {
         const closePromise = new Promise(resolve => socket.once('close', resolve));
         socket.end();
         await closePromise;
+
+        // should still have client - grace period has not expired
+        assert(manager.hasClient('foobar'));
+
+        // wait past grace period (1s)
+        await new Promise(resolve => setTimeout(resolve, 1500));
         assert(!manager.hasClient('foobar'));
-    });
+    }).timeout(5000);
 });